Immunoassays are known in which target proteins contained in samples are measured with antibodies capable of specifically binding to the target proteins by using antigen-antibody reaction. For example, sandwich immunoassays are commonly used in which samples containing target proteins are brought into contact with supports such as microtiter plates, magnetic particles, on which primary antibodies capable of binding to the target proteins are immobilized, so as to allow the binding of the target proteins to the primary antibodies; secondary antibodies capable of binding to the same target proteins are then allowed to bind to the target proteins; and labelling substances conjugated to the secondary antibodies are detected.
Immunoassays using as supports microtiter plates, magnetic particles and the like are carried out in a volume of around several hundreds of microliters and therefore use a large amount of antibodies directed to the target proteins. Thus, they may require a large amount of expensive antibodies and also be difficult to measure multiple target proteins. In addition, the immunoassays have other disadvantages: they generally take several hours to react target proteins with the antibodies; they require very laborious operations such as solution exchange, washing and others due to the multistep reaction; and they are not highly sensitive.
A microchannel device has been known which comprises a glass or plastic capillary embedded therein in at least a portion of the channel (Japanese Patent No. 4073023: Patent Literature 1). It is also known that such a capillary for a microchannel device can be used for an immunoreaction.
By using the technologies, capillary immunoassays have been recently developed in which antigen-antibody reactions are carried out in square capillaries having bores of about 100 μm on a side (for example, Henares T. G. et al., Analytica Chimica Acta 589 (2007) p. 173-179: Non-patent Literature 1). These use sandwich immunoassays in which the presence of antigens is detected with primary and secondary antibodies by passing antigen solutions, the enzyme-conjugated secondary antibodies and substrate solutions sequentially through the capillaries to which the primary antibodies are immobilized on the inner wall surfaces.
Such small volume measurement systems made it possible to reduce the amount of antibodies to be required.
In these methods, however, after flowing the antigen solutions and the enzyme-conjugated secondary antibodies through the capillaries, a washing step is required for washing off unbound antigens and secondary antibodies. This makes the capillary immunoassay methods laborious.